udp: Resolve NULL pointer dereference over flow-based vxlan device

While testing an OpenStack configuration using VXLANs I saw the following
call trace:

 RIP: 0010:[<ffffffff815fad49>] udp4_lib_lookup_skb+0x49/0x80
 RSP: 0018:ffff88103867bc50  EFLAGS: 00010286
 RAX: ffff88103269bf00 RBX: ffff88103269bf00 RCX: 00000000ffffffff
 RDX: 0000000000004300 RSI: 0000000000000000 RDI: ffff880f2932e780
 RBP: ffff88103867bc60 R08: 0000000000000000 R09: 000000009001a8c0
 R10: 0000000000004400 R11: ffffffff81333a58 R12: ffff880f2932e794
 R13: 0000000000000014 R14: 0000000000000014 R15: ffffe8efbfd89ca0
 FS:  0000000000000000(0000) GS:ffff88103fd80000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000488 CR3: 0000000001c06000 CR4: 00000000001426e0
 Stack:
  ffffffff81576515 ffffffff815733c0 ffff88103867bc98 ffffffff815fcc17
  ffff88103269bf00 ffffe8efbfd89ca0 0000000000000014 0000000000000080
  ffffe8efbfd89ca0 ffff88103867bcc8 ffffffff815fcf8b ffff880f2932e794
 Call Trace:
  [<ffffffff81576515>] ? skb_checksum+0x35/0x50
  [<ffffffff815733c0>] ? skb_push+0x40/0x40
  [<ffffffff815fcc17>] udp_gro_receive+0x57/0x130
  [<ffffffff815fcf8b>] udp4_gro_receive+0x10b/0x2c0
  [<ffffffff81605863>] inet_gro_receive+0x1d3/0x270
  [<ffffffff81589e59>] dev_gro_receive+0x269/0x3b0
  [<ffffffff8158a1b8>] napi_gro_receive+0x38/0x120
  [<ffffffffa0871297>] gro_cell_poll+0x57/0x80 [vxlan]
  [<ffffffff815899d0>] net_rx_action+0x160/0x380
  [<ffffffff816965c7>] __do_softirq+0xd7/0x2c5
  [<ffffffff8107d969>] run_ksoftirqd+0x29/0x50
  [<ffffffff8109a50f>] smpboot_thread_fn+0x10f/0x160
  [<ffffffff8109a400>] ? sort_range+0x30/0x30
  [<ffffffff81096da8>] kthread+0xd8/0xf0
  [<ffffffff81693c82>] ret_from_fork+0x22/0x40
  [<ffffffff81096cd0>] ? kthread_park+0x60/0x60

The following trace is seen when receiving a DHCP request over a flow-based
VXLAN tunnel.  I believe this is caused by the metadata dst having a NULL
dev value and as a result dev_net(dev) is causing a NULL pointer dereference.

To resolve this I am replacing the check for skb_dst(skb)->dev with just
skb->dev.  This makes sense as the callers of this function are usually in
the receive path and as such skb->dev should always be populated.  In
addition other functions in the area where these are called are already
using dev_net(skb->dev) to determine the namespace the UDP packet belongs
in.

Fixes: 63058308 ("udp: Add udp6_lib_lookup_skb and udp4_lib_lookup_skb")
Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>